The increase in driving, known to economists as the rebound effect, is factored in to the Obama administration’s calculation that the new standards will save 1.8 billion barrels of oil, with an assumption that it’s a rather modest 5%. That means a decrease of X% in fuel needed to drive a mile will lead to an increase of 0.05X% in driving.

Earlier calculations by the National Highway Traffic Safety Administration assumed a higher rebound effect of 15% (earlier this year) and 20% (in 2006). But several economists say the lower level is appropriate, given a declining rebound effect as people earn more and care more about time lost to driving than about fuel costs.

Among these studies are those mentioned in the column by David Greene, a senior scientist at Oak Ridge National Laboratory, and another by Kenneth Small, an economist at the University of California, Irvine, and Kurt Van Dender, an economist for the Organisation for Economic Co-operation and Development. Kenneth Gillingham, a graduate student at Stanford University conducting his own study, has found that the rebound effect will be 10% or lower in the future.

This figure has been the subject of lobbying in the past. Jim Kliesch, a senior engineer at the Union of Concerned Scientists, argued for a figure of 10% in 2006; General Motors pushed for 20%, the figure ultimately chosen. When told EPA used 5% for its latest calculations, Mr. Kliesch said, “That’s great to hear, and consistent with the administration’s stated position that they’re going to follow the science when setting rules.”

As for GM, which has received billions in federal bailout funds, a spokesman declined to comment on its earlier rationale for a 20% figure, emphasizing that “GM is fully committed to this new approach.”

The federal government’s assumed rebound effect has a big effect on its assessment of the environmental impact of fuel-efficiency regulations. For instance, when NHTSA used a figure of 15% in March for model years 2011 to 2015, it also calculated what the projected fuel savings would have been for a rebound effect of 10% and 20%. The former yielded fuel savings of 630 billion gallons, 28% more than the latter.

Interestingly, when scientists have tested whether drivers react to changes in fuel efficiency as they do to changes in gas prices, they haven’t been able to demonstrate a statistically significant rebound effect. There are a host of possible explanations, including that drivers who opt for more fuel-efficient cars are doing so in part out of environmental consciousness, and would reject more driving on that basis.

This finding could also reflect a failing of the aggregated government data to get at individual driver behavior. No one has conducted a longitudinal study tracking individual drivers over many years, which would be useful to “tease out from individual consumer behavior how much improvement in automobile efficiency affects their use and transportation choices,” says Phil Hanser, an economist with the Brattle Group.

The rebound effect isn’t the only numerical complication with the new rules. The stated goals for fuel efficiency are much greater than the true fuel efficiency the average new car will produce in that year; the EPA now requires that manufacturers adjust that lab-derived figure downward for their window stickers to reflect real-world driving, but the higher figure is used in legislation for consistency.

What do you think? How much more do people drive when they upgrade to more-fuel-efficient vehicle? How much environmental impact will the new standards have? How important is fuel efficiency to you when buying a vehicle? Please let me know in the comments.

Further reading: Miles-per-gallon measures can lead to some odd math. Othershavedownplayed the rebound effect as it pertains to the new fuel-efficiency standards. The aforementioned studies of the effect are based on government data on driving, which isn’t exact.

About The Numbers

The Wall Street Journal examines numbers in the news, business and politics. Some numbers are flat-out wrong or biased, while others are valid and help us make informed decisions. We tell the stories behind the stats in occasional updates on this blog.